Calculating my due date and I believe my Dr is wrong

Ok, so I'm supposedly in my 10th week. But according to my last ovulation period, I am calculating my due date to atleast three weeks after the date the Dr ks assuming. He's basing it on my last period but I had my last period (it was messy, heavy and painful- definitely not spotting). I'm so annoyed because even his machine is giving a different date.

Should be due in mid Jan, he says I'm due end of December...